Final answer:
The opening of the parachute increases air resistance, which in turn decreases the net force on the parachutist. This causes a reduction in the parachutist's acceleration according to Newton's second law, leading to a slower descent speed.
Step-by-step explanation:
When the parachutist is much nearer to the ground, they open their parachute which significantly increases the air resistance they experience. A gravitational force, which remains constant, pulls the parachutist towards the ground, and before the parachute is opened, the skydiver accelerates due to this force. However, opening the parachute causes a large increase in drag force (air resistance), which opposes the force of gravity. Eventually, the drag force becomes nearly equal to the gravitational force resulting in a significant reduction in the parachutist's acceleration. The net force acting on the parachutist becomes minimal, leading to a decrease in speed as they approach a new, much lower terminal velocity.
The change in speed can be explained by Newton's second law of motion, which states that the acceleration of an object is directly proportional to the net force acting on the object and inversely proportional to its mass. Since the net force decreases with the opening of the parachute, the acceleration of the parachutist decreases, causing them to slow down. This transformation of forces ensures that the parachutist lands safely by decreasing their descent speed.